Constant current power supply



Dec. 10, 1957 o PRE|$|G CONSTANT CURRENT POWER SUPPLY Filed Oct. 29, 1954 -III ..I DSEPH El. PREISIE BY Z INVENTOR.

nitcd States Patent Ofiice 2,816,26 l Patented Dec. 10, 1957 coNsrANr CURRENT POWER SUPPLY Joseph 0. Preisig, Trenton, N. L, assignor to Radio Corporation of America, a corporation of Deiaware Application Uctoher 29, 1954, Serial No. 465,691 11 Claims. (Cl. 321-43) This invention relates generally to constant current power supplies, and more particularly to power supplies of the type adapted to provide substantially constant current at a relatively high voltage, in the order of kilovolts. While neither specifically nor exclusively limited thereto, the constant current power supply of the present invention is particularly useful as a power supply for monochrome or color television receivers, and the like.

It has been proposed to regulate conventional power supplies by sampling the output voltage with a bleeder resistance and feeding back a sample voltage degeneratively to a variable impedance device either in series or in parallel with the load on the power supply. While such prior art power supplies provide adequate regulation for relatively low output voltages, they present serious problems in construction when adapted to provide output voltages in the order of kilovolts. For higher output voltages, the bleeder resistance may generate a large amount of heat and thereby waste a large amount of the output power available, in addition to causing instability of the power supply. A bleeder resistance adapted to withstand a voltage of 20 kilovolts thereacross, for example, is also a relatively expensive component. The use of such circuit components in the control circuit of the power supply also requires relatively expensive insulation for the control circuit.

Accordingly, it is a principal object of the present invention to provide an improved constant current power supply adapted to overcome the aforementioned objections and disadvantages.

Another object of the present invention is to provide an improved power supply adapted to provide a substantially constant current at output voltages in the order of kilovolts, while employing a control circuit requiring insulation necessary to withstand only a few hundred volts.

A further object of the present invention is to provide an improved constant current power supply wherein the use of a relatively expensive and power consuming high voltage bleeder is eliminated.

It is still a further object of the present invention to provide an improved constant current power supply that is relatively simple in construction and operation, economical to manufacture and yet highly efiicient in use.

These and, perhaps, other objects of the present invention are attained in an improved constant current power supply of the type wherein a stepped-up A.-C. or pulsating voltage is rectified and applied across a load. A variable impedance device, such as a triode tube, is connected in shunt relationship with the load. Means are provided to vary the impedance of the variable impedance device inversely with variations in the impedance of the load whereby the total impedance presented to the power supply remains substantially constant. To this end, the charging capacitor of the usual power supply filter is connected in series with an impedance element, the

charging capacitor and impedance element comprising a series circuit connected across the source of unregulated unidirectional voltage. With this arrangement, the charging current flowing to the charging capacitor of the filter is proportional to the current flowing to the load. The A.-C. voltage developed across the impedance element, as a result of the charging current to the charging capacitor, is rectified and applied degeneratively to the variable impedance device so that impedance of the device will vary inversely with the impedance of the load, and thereby provide a substantially constant total impedance to the power supply. The impedance element may comprise either a resistance or an inductance element.

The novel features of the present invention, as well as the invention itself, both as to its organization and method of operation will be understood in detail from the following description when considered in connection with the accompanying drawing in which similar reference characters designate similar elements; and in which:

Fig. 1 is a schematic drawing of an improved constant current power supply, in accordance with the present in vention,

Fig. 2 is a preferred embodiment of the constant current power supply, in accordance with the present invention, and

Fig. 3 is a modification of the constant current power supply, in accordance with the present invention.

Referring now particularly to Fig. 1, there is shown an electronic constant current power supply, in accordance with the present invention, employing a rectifier, such as a diode tube 10. The anode of the tube It) is connected to a point of reference potential, such as ground, through a secondary winding 12 of a power transformer 14. The power supply of Fig. 1 derives its input energy from a source of A.-C. or pulsating voltage (not shown) adapted to be applied across a primary winding 16 of the transformer 14. The cathode of the tube 10 is connected to a positive output terminal 18 for supplying current to a load Ztl, connected between the output terminals 18 and ground, and represented herein as a resistor.

Means are provided to maintain the total current through the power supply substantially constant with changes in the impedance of the load 20. To this end, a control tube 22 such as a triode, is connected in shunt relationship with the load 20. The anode of the tube 22 is connected to the cathode of the tube 10, and the cathode of the tube 22 is connected to ground. A charging capacitor 24, that may be considered as a part of the filtering network for the unregulated rectified voltage appearing between the cathode of the tube 19 and ground, has one terminal or plate connected to the cathode of the tube 10 and the other terminal or plate connected to ground through an impedance element, such as a variabic resistor 26. It will now be understood that a portion of the current flowing from the cathode of the tube 10 to the load 20 is diverted through the circuit comprising the capacitor 24- and the resistor .26.

Means are provided to rectify the pulsating voltage across the resistor 26, resulting from the charging current to the capacitor 24. To this end, a rectifier 28, which may be a dry type rectifier such as a germanium diode, has its cathode connected to the junction of the capacitor 24 and the resistor 26, and its anode to the control grid of the control tube 22. The anode of the diode 28 is also connected to ground through a small filter capacitor 30. The capacitor 30 may be considered the integrating capacitor of the conventional 1r-type filter.

The operation of the power supply illustrated in Fig. 1, in accordance with the present invention, will now be described. Let it be assumed that the load 20 varies in impedance, as where the load represents a cathode ray tube whose impedance varies with the amplitude and the polarity of the input signals applied thereto. The source of A.-C. voltage (not shown), applied to the priinary winding 16 of the transformer 14, is stepped-up by the secondary winding 12, and rectified by the rectifier tube 10. The output voltage, between the output terminals 18 and ground, may be in the neighborhood of 20 lrilovolts. The current tiowing through the load 20 will vary with changes in the impedance of the load 20. Let it now be assumed that the impedance of the load 20 increases, whereby less current will tend to flow through the load 20. Since the current flowing to the load 20 is proportional to the charging current flowing to the circuit comprising the charging capacitor 24 and the resistor 26, this charging current will also decrease. Consequently, the pulsating voltage appearing across the resistor 26 will also decrease. The pulsating voltage of decreased amplitude appearing across the resistor 26 is rectified by the diode 28, and applied to the control grid of the control tube 22 as a positive-going voltage, whereby to decrease the impedance of the control tube 22. It will now be understood that an increase in the impedance of the load 20 gives rise to a set of conditions resulting in a decrease in the impedance of the control tube 22 connected in shunt relationship with the load 20. Thus, it will be understood that the total impedance, comprising the load 20 and the control tube 22 connected in parallel therewith, tends to remain substantially constant since their impedances vary inversely with respect to each other with changes in the load. Under these conditions, the total impedance seen by the power supply remains substantially constant, whereby the current supplied by the power supply will be substantially constant. From the foregoing explanation, it is obvious that a decrease in the impedance of the load 2%) will give rise to a reverse set of conditions, whereby the impedance of the control tube 22 will increase and the total current supplied by the power supply will tend to remain substantially constant.

Referring now to Fig. 2, there is shown a constant current power supply substantially similar to Fig. 1 except for the coupling means for coupling the rectifier 28 to the impedance element in series with the charging capacitor 24. In Fig. 2, the charging capacitor 24 is connected to ground through an impedance element comprising a primary winding 32 of a transformer 34. The diode 28 is adjustably coupled to the primary winding 32 by means of a secondary winding 36 connected between the cathode of the rectifier 28 and the cathode of the control tube 22. Where necessary, the cathode of the control tube 22 may be maintained at a fixed voltage with respect to ground by means of a voltage regulator tube 38, or a battery (not shown), or any other suitable means well known in the art.

The operation of the constant current power supply illustrated in Fig. 2 will now be described. Let it be assumed that the impedance of the load 20 is suddenly decreased so that more current is drawn therethrough. Since the charging current flowing in the circuit comprising the capacitor 24 and the primary winding 32 is proportional to the current taken by the load, the pulsating voltage developed across the primary winding 32 will increase. Since the diode 28 is coupled to the primary winding 32 by means of the secondary winding 36, which may step-up the primary voltage, the voltage rectifiedby the diode 28 will be a negative-going voltage. This last mentioned negative-going voltage is applied to the control grid of the control tube 22 whereby the impedance of the control tube 22 is increased. Thus, it is seen that a decrease in the impedance of the load 20 results in an increase in impedance of the control tube 22 shunted thereacross. It will also be understood that an increase in theimpedance of the load 20, in Fig. 2, will result in a reverse set ofconditions whereby the grid of the variable impedance impedance of the control tube will decrease. It is obvious that with proper values for the components of the power supply the changes in the impedance of the load may be substantially balanced by compensating changes in the impedance of the shunt control tube. With this arrangement, substantially the total load presented to the power supply, that is, the total load represented by the load 20 and the impedance of the control tube 22 will remain substantially constant so that the current supplied by the power supply will also be substantially constant.

Referring now to Fig. 3, there is shown a constant current power supply, in accordance with the present invention, substantially similar to the one shown and described in Fig. l. The power supply of Fig. 3 differs from that of Fig. l in that in Fig. 3 the charging capacitor 24 is connected to ground through an impedance element comprising a variable inductance 40. Also in Fig. 3, the cathode of the rectifier tube 10 is connected to the output terminal 18 through a filter inductance 42, and the output terminal 18 is connected to ground through a filter capacitor 44. The inductance 42 and capacitor 44 may be omitted where relatively slight ripple voltages are not objectionable, without seriously affecting the op eration of the power supply. The operation of the power supply of Fig. 3 is substantially similar to that of Fig. 1 when the inductance 40 is considered as an impedance element whose function is substantially similar to that of the resistor 26 in Fig. l.

The resistor 26 in Fig. l, the coupling between the primary and secondary windings 32 and 36 of the transformer 34, in Fig. 2 and the inductance 40 in Fig. 3 are made variable for the purpose of controlling the pulsat ing voltage developed across these elements whereby to provide the grid of the control tube 22 with the proper rectified operating voltage.

Thus, there is shown and described herein, in accord ance with the objects of the present invention, a power supply adapted to provide a total constant current regardless of changes in the load. The impedance of the control tube, connected in shunt relationship with the load, is controlled by voltages applied thereto degeneratively with changes in the impedance of the load. It will be noted that these last mentioned voltages are not derived from a bleeder resistance that is wasteful of power, generates excessive heat, and causes an instability of the output voltage. The voltages applied to the control grid of the tube 22 are derived from a relatively low voltage developed across an impedance element comprising either a variable resistance, a transformer winding, or a variable inductance. These variable impedance elements need be insulated for, and constructed to withstand, only a few hundred volts even though the power supply may be designed to provide an output in the neighborhood of 20,000 volts, and more. Thus, the problem of reducing the cost, power consumption, and heat dissipation of bleeder resistors in high voltage power supplies is substantially solved. Since the rectifier providing the voltage to the control control tube need only rectify a relatively low voltage, this portion of the control circuit of the power supply may also be insulated for only a few hundred volts instead of for the total voltage provided at the output of the power supply.

What is claimed is:

1. Power supply apparatus comprising a first rectifier having an anode and a cathode, a point of reference potential in said apparatus, means to apply a source of alternating voltage between said anode and said point, means to apply a load between said cathode and said point, a variable impedance device, means connecting said device between said cathode and said point, a capacitor, an im pedance element, means connecting said capacitor and impedance eledent in series with each other and between said cathode and said point, a second rectifier, means coupling said sccondrectifier to said impedance element to rectify pulsating voltages thereacross, and means to apply said rectified voltages to said device to vary the impedance of said device with changes in impedance of said load.

2. Power supply apparatus comprising a first rectifier having an anode and a cathode, a point of reference potential in said apparatus, means to apply a sorrce of alternating voltage between said anode and said point, means to apply a load between said cathode and said point, a variable impedance device, means connecting said device between said cathode and said point, a capacitor, an impedance element, means connecting said capacitor and impedance element in series with each other and between said cathode and said point, a second rectifier, means coupling said second rectifier to said impedance element to rectify pulsating voltages thereacross, means to apply said rectified voltages to said device to vary the impedance of said device with changes in impedance of said load, said impedance element comprising a resistor, and said coupling means comprising a connection between said resistor and said second rectifier.

3. Power supply apparatus comprising a first rectifier having an anode and a cathode, a point of reference potential in said apparatus, means to apply a source of alternating voltage between said anode and said point, means to apply a load between said cathode and said point, a variable impedance device, means connecting said device between said cathode and said point, a capacitor, an impedance element, means connecting said capacitor and impedance element in series with each other and between said cathode and said point, a second rectifier, means coupling said second rectifier to said impedance element to rectify pulsating voltages thereacross, means to apply said rectified voltages to said device to vary the impedance of said device with changes in impedance of said load, said impedance element comprising a primary winding of a transformer, and said coupling means comprising a secondary winding of said transformer, said secondary winding having one end connected to said second rectifier and the other end connected to said device.

4. Power supply apparatus comprising a first rectifier having an anode and a cathode, a point of reference potential in said apparatus, means to apply a source of alternating voltage between said anode and said point, means to apply a load between said cathode and said point, a variable impedance device, means connecting said device between said cathode and said point, a capacitor, an impedance element, means connecting said capacitor and impedance element in series with each other and between said cathode and said point, a second rectifier, means coupling said second rectifier to said impedance element to rectify pulsating voltages thereacross, means to apply said rectified voltages to said device to vary the impedance of said device with changes in impedance of said load, and said impedance element comprising an inductance.

5. Power supply apparatus comprising a first rectifier having an anode and a cathode, a point of reference potential in said apparatus, means to apply a source of alternating voltage between said anode and said point, means to apply a load between said cathode and said point, a variable impedance device, means connecting said device between said cathode and said point, a capacitor, an impedance element, means connecting said capacitor and impedance element in series with each other and between said cathode and said point, a second rectifier, means coupling said second rectifier to said impedance element to rectify pulsating voltages thereacross, means to apply said rectified voltages to said device to vary the impedance of said device with changes in impedance of said load, said impedance element comprising an inductance, and said coupling means comprising a connection between said inductance and said second rectifier.

6. In a power supply of the type wherein an A.-C. input voltage is stepped-up, rectified and applied across a pair of output terminals for supplying current to a load adapted to be connected across said terminals, the combination therewith ofa variable impedance device connected across said terminals, a capacitor having one side connected to one of said terminals, an impedance element having one end connected to the other of said terminals, means connecting the other side of said capacitor to the other end of said impedance element, a rectifier, means coupling said rectifier to said impedance element to rectify pulsating voltages thereacross, and means to apply said rectified pulsating voltages to said device.

7. In a power supply of the type wherein an A.-C. input voltage is stepped-up, rectified and applied across a pair of output terminals for supplying current to a load adapted to be connected across said terminals, the combination therewith of a variable impedance device connected across said terminals, a capacitor having one side connected to one of said terminals, an impedance element having one end connected to the other of said terminals, means connecting the other side of said capacitor to the other end of said impedance element, a rectifier, means coupling said rectifier to said impedance element to rectify pulsating voltages thereacross, means to apply said rectified pulsating voltages to said device, and said impedance element comprising a resistor.

8. In a power supply of the type wherein an A.-C. input voltage is stepped-up, rectified and applied across a pair of output terminals for supplying current to a load adapted to be connected across said terminals, the combination therewith of a variable impedance device connected across said terminals, a capacitor having one side connected to one of said terminals, an impedance element having one end connected to the other of said terminals, means connecting the other side of said capacitor to the other end of said impedance element, a rectifier, means coupling said rectifier to said impedance element to rectify pulsating voltages thereacross, means to apply said rectified pulsating voltages to said device, said impedance element comprising a resistor, and said coupling means comprising a connection between said resistor and said rectifier.

9. In a power supply of the type wherein an A.-C. input voltage is stepped-up, rectified and applied across a pair of output terminals for supplying current to a load adapted to be connected across said terminals, the combination therewith of a variable impedance device connected across said terminals, a. capacitor having one side connected to One of said terminals, an impedance element having one end connected to the other of said terminals, means connecting the other side of said capacitor to the other end of said impedance element, a rectifier, means coupling said rectifier to said impedance element to rectify pulsating voltages thereacross, and means to apply said rectified pulsating voltages to said device, said impedance comprising a first winding of a transformer, said coupling means comprising a second winding of said transformer, and means connecting one end of said second winding to said rectifier and the other end of said second winding to said device.

10. In a power supply of the type wherein an A.-C. input voltage is stepped-up, rectified and applied across a pair of output terminals for supplying current to a load adapted to be connected across said terminals, the combination therewith of a variable impedance device connected across said terminals, a capacitor having one side connected to one of said terminals, an impedance element having one end connected to the other of said terminals, means connecting the other side of said capacitor to the other end of said impedance element a rectifier, means coupling said rectifier to said impedanc nent LO rectify pulsating voltages thereacross, means to apply said rectified pulsating voltages to said device, and said impedance element comprising an inductance.

11. In a power supply of the type wherein an AC. input voltage is stepped-up, rectified and applied across a pair of output terminals for supplying current to a load adapted to be connected across said terminals, the com- 7 3 bination therewith of a variable impedance device conmeans comprising a connection between said inductance nected across said terminals, a capacitor having one side and said rectifier. connected to one of said terminals, an impedance element 0 having one end connecetd to the other of said terminals, Rfiemmis Cited In the file of thls Patent means connecting the other side of said capacitor to the 5 UNITED STATES PATENTS other end of said impedance element, a rectifier, means coupiing said rectifier to said impedance element to rectify R g 3; 3 v, a J J A 1 pulsating voltn es thereacross, means to apply said recti 2,434,069 G01 dberg M- Jan- 1948 fied pulsating voltages to said device, said impedance element comprising an inductance, and said coupling 10 2609446 Michels Sept'21952 

